Current limiting fuse



H. w. MIKULECKY 3,243,552

CURRENT LIMITING FUSE March 29, 1966 2 Sheets-Sheet 1 Filed sept. e,1964 e fl,

I rif-.nf A, /l, .s

BY TL 3,243,552 CURRENT LIMITING FUSE Harvey W. Mikulecky, Racine, Wis.,assignor to McGraw- Edison Company, Milwaukee, Wis., a corporation ofDelaware Filed Sept. 8, 1964, Ser. No. 394,957 13 Claims. (Cl. 200-120)This invention relates to fuses and, more particularly, to fuses of thecurrent limiting type which limit the flow of current in an electricalcircuit under short circuit conditions to a substantially smaller valuethan the available short circuit current of the circuit.

Current limiting fuses conventionally comprise a fusible elementembedded in a granular inert material of high dielectric strength suchas sand or linely divided quartz. Usually the fusible element is in theform of one or more thin conductors of silver Wou-nd on a supportingcore, or spider, of high temperature resistant ceramic material. Whensubjected to current of fault magnitude, the fusible element v-attainsfusing temperature and vaporizes, whereby arcing occurs and the metalvapors rapidly expand to many times the volume originally occupied bythe fusible velement and are thrown into the spaces between the granulesof inert filler material where they condense and are no longer availablefor current conduction. The current limiting elfe-ct results from theinteraction of the metal vapors and the'i-nert granular materialsurrounding the fusible element. The physical contact between the hotarc and the relatively cool `granules causes a rapid transfer of heatfrom the arc to the granules, thereby dissipating most of the arc energywith very little pressure buildup within the fuse enclosure. The vaporsof silver have relatively low conductivity unless their temperature isparticularly high, and the temperature lof the silver vapors is rapidlyreduced by the quartz sand filler .until the vapors will not support allow of current. Consequently, a high resistance is, in effect, insertedi-nto the path of the current and initially limits the current to amagnitude which is only a small fraction of that avail,- able in thecircuit.

The quartz sand particles in the immediate vicinity of the arc fuse andbecome partial conductors at the high temperature of the arc and form-a`fulgurite, or semiconductor. The fulgurite resulting from fusion andsintering of the quartz sand particles is in the nature of a glass body,and as it cools it loses its conductivity and becomes an insulator.

High voltage, high amperage current limiting fuses used to control faultcurrents of high magnitude conventionally employ fusible elements ofsilver ribbon having serially related portions of relatively small crosssectional area and intermediate portions of relaitvely largel crosssectional area, for example, a silver ribbon provided with a pluralityof circular spaced apart perforations which determine the portions wherefusion of the fusible element is initiated on currents of shortcircuitmagnitude. The perforations form portions of reduced crosssectional larea which limit the peak arc voltage and make it possible todistribute the thermal duty Iof the arc quenching granular materialrelatively evenly over the entire filler body. If such a fuse is subjectto fault current of high magnitude, all of the portions of small crosssectional area fuse and vaporize almost simultaneously, resulting in theformation of arclets in series and controlling the transient voltageacross the fuse. When subjected to small United States Patent O3,243,552 Patented Mar. 29, 1 966 ICC protracted fault currents, the arcgap first formed is generally progressively enlarged by vaporization ofthe silver element until the gap is of suicient length to eifect finalinterruption of the circuit .and consequently the fulgurite is generallycontinuous. When interruption of small overload currents results inarcing over a plurality of cycles, the arc energy tends to be large. Therelatively large arc energy and the dissipation of additional heatresulting from 12R losses caused by the flow of follow current throughthe fulgurite combine to delay the cooling of the central portion of thefulgurite which remains partially conductive, and -only the end portionsof the fulgurite, where the arc contacts the relatively cool fillerparticles, tend to interrupt the arc. Most of the voltage appears acrossthe ends of the fulgurite, which are of higher resistance than the hotcentral portions thereof, and tends to ilashover the hot gases, andconsequently reignition of the fusible element and post interruptionfailure frequently occur when current limiting fuses control overloadcurrents of small magnitude.

It is an object of the invention to provide a current limiting fuse forcontrolling both currents of overload proportions and of short circuitproportions which has improved means to prevent re-ignition of the .arcwhen clearing relatively low magnitude currents.

It is a further object of the invention to provide a current limitingfuse for interrupting both large and small fault currents which hasmeans to form a series of arc gaps or fulgurites rather than acontinuous fulgurite when clearing prolonged, low magnitude overloadcurrents.

These and other objects and advantages of the invention will be morereadily apparent from the following detailed description when taken inconjunction with the Vaccompanying drawing wherein:

FIG. 1 is a sectional view through a current limiting fuse embodying theinvention;

FIG. 2 is a view taken on line 2 2 of FIG. 1;

FIG. 3 is a view taken on line 3 3 of FIG. 1;

FIGS. 4a-4f are schematic views illustrating the sequential operation ofthe fuse of FIG. l when interrupting small magnitude fault currents;

FIGS. 5 and 6 are schematic views of alternative embodiments of theinvention illustrating the main and auxiliary fusible elements in linearform rather than helically wound -as in the actual construction and asshown in FIG. l; and

FIG. 7 is a perspective view of the arcing electrode utilized in theembodiment of FIG. 1.

Referring to the drawing, a tubular enclosing casing 10 for a currentlimiting fuse is constructed of suitable insulating material such asglass liber impregnated with epoxy resin, glass, or liber. A metallicend piece 11R is secured on the right end of casing 10 by means of anysuitable seal such as epoxy cement 12 and radially extending pins 14(see FIG. 3) protruding through the casing 10 and into holes in endpiece 11R. A metallic end piece 11L may have external threads engaginginternal threads formed near the left end of casing 10. A metallic hingeassembly 16 may be secured to end piece 11R shown at the right end offuse casing 10 by screws 17 engaged within threaded apertures in the endpiece 11R. The end piece 11L shown at .the left end of fuse casing 10has a smaller diameter portion 18 and an axial bore 19. A tubularmetallic terminal member 20 extends into axial bore 19 with a force titand is rigidly secured to end piece 11L by pins 21 extending radiallythrough tubular member and smaller diameter portion 18 of end piece 11L.Terminal member 20 may be adapted to fit within a stationary contact jawof an electrical switch, :and an insulating member 22 provided with aneye (not shown) for receiving a hookstick may be secured in the end ofterminal member 20 by suitable means such as epoxy cement. End plates 23are disposed against the internal surface of end pieces 11L and 11R andare secured thereto by screws 24. Each end plate 23 has a plurality ofradially extending tabs 25 adjacent its outer periphery which may bebent down and form terminals to which the fusible conductors can beconnected.` An elongated insulating core, or spider 26, is 4axiallymounted wi-thin casing 10. The ends of spider 26 are affixed to metallicend plates 23 by suitable means such as epoxy cement.

Spider 26 is of generally star-shaped cross section having a pluralityof radially protruding, peripherally spaced apart, longitudinallyextending fins 32. Each iin 32 has a plurality of depressions 33 ofsemi-cruciform configuration spaced apart longitudinally of spider 26forming longitudinally spaced apart raised shoulders 34.

The depressions 33 of peripherally successive ns 32 are progressivelystaggered in a direction longitudinal of the spider 26 so that theperipherally successive depressions 33 define a continuous helical pathand `the peripherally successive raised shoulders 34 form support meansof helical configuration for a circuit interrupting main fusible element36 which is usually of silver or copper and norm-ally carries all ofthecurrent and as disclosed in FIGS. 1 and 2 comprises a pair of parallelribbons 37 of suit-able fusible material such as silver helically woundso as to contac-t only the peripherally spaced apart raised shoulders 34of spider 26 and being electrically connected in parallel betweenmetallic end pieces 11. The rends of the ribbons 37 are soldered to tabs25 on end plates 23.

Spider 26 may be of inert ceramic material such as porcelain, but itpreferably is of an electrical insulating material adapted to evolve gasin the presence of an arc, as disclosed in my oopending applicationSerial No. 313,- 640 filed October 10, 1963, having suiicient mechanicalStrength to be self supporting, and being capable of withstandingtemperatures up to 250 F. continuously without degradation andtemperatures up to 500 F. for periods up to one hour without excessivedegradation or decomposition. Spider 26 may be of a molded thermosettingcomposition comprising a water insoluble binder and an anti-trackingsubstance selected from the class consisting of the hydrates and oxidesof aluminum and magnesium. The composition may also include otherfillers such as mica, glass, liber, asbestos, or silica, and onesuitable material comprises approximately 60 percent aluminum hydratefiller, 20 percent melamine resin binder, and approximately 20 percentasbestos.

When the fuse is required to control currents of short circuitproportions, the silver ribbons 37 of the main fusible element 36 may beprovided with a plurality of circular perforations, or holes, 39 spacedapart :along the length thereof which determine the points where fusionof the element is initiated when the fault current and its rate of riseare high. The perforations 39 form portions tof reduced cross sectionalarea so that each ribbon 37 has :a number of serially related portionsof relatively small cross sectional and intermediate areas of relativelylarge cross sectional area. Beads 40 of low melting temperature alloysuch as :tin-lead solder are in intimate contact with the main silverribbons 37 preferably adjacent the midpoint thereof. At melting currentsflowing for prolonged periods, the fusible ribbons 37 become hot enoughto melt the alloy bodies 40, and the amalgamation of the silver andalloy causes a hot spot with high enough resistance to melt the ribbon37 at this point., This construction, known as the M effect, allows thefusible ribbon 37 to melt at a temperature in the 400-600 F. range whensubjected over a long period of time to low magnitude currents ascompared to the 1760 F. melting temperature for pure silver. It will beappreciated that on currents of short circuit magnitude, the alloyelement has little or no effect and the silver elements 37 vaporize atthe fusion temperature for the silver.

An auxiliary fusible element 41 is wound on spider 26 in radially inwardspaced relation from the main element 36 and in the helical path definedby the depressions 33. Auxiliary element 41 may be a copper or a silverribbon, a copper or a silver wire, a plurality of silver wires 42, asshown in the drawing, or a resistance Wire, and is electricallyconnected at its ends by suitable means such as solder t-o metallicclip, arc gap electrodes 43 each of which tits over one of the raisedshoulders 34 of spider 26. The ends of clip electrodes 43 areresiliently urged against the raised shoulder 34 to prevent movement ofthe electrode 43 relative to spider 26. The arc gap electrodes 43 arelocated at points approximately midway between the low meltingtemperature alloy bead 40 and the ends of the main fusible ribbons 37,and the arc gap electrodes 43 are spaced radially inward from the mainribbon 37 so that an air gap of approximately 1A inch to 1/32 inchexists between the electrodes 43 and the main ribbons 37. The arc gapelectrodes 43 have sufficient mass to withstand arcing Withoutdeformation or melting over prolonged periods until the auxiliaryelement 41 clears the fault. Depending tab portions 45 extending in adirection away from the main ribbons 37 are provided on the arc gapelectrodes 43 to which the ends of the auxiliary element 41 are attachedby suitable means such as solder so that the arc cannot easily contactthe auxiliary element 41 and cause premature melting thereof.

Auxiliary fusible element 41 is selected so that the ratio of theaverage 100 second melting current of the main element 36 to the average100 second melting cur-rent for the auxiliary element 41 is in the rangeof 2 to 6, and preferably this ratio is appnoximately four. Stated inanother manner, auxiliary fusible element 41 is selected to have aminimum melting current sufficiently less than that of the main element36 so that, when the minimum melt current is reached for the mainelement 36, good low current clearing characteristics exist for theauxiliary element 41.

An indicator wire 47 of suitable material having higher resistance thansilver such as tungsten or Nichrome is connected at one end to the arcgap electrode 43R adjacent hinge assembly 16 at the right end of thefuse as shown in the drawing. The indicator wire 47 is Wound in thehelical path delined by the depressions 33 and extends through anopening 48 in the right end of spider 26, through axial opening in aninsulating disc 49, the end plate 23, and the end piece 11R, and issecured by suitable means such as solder to the bottom wall 50 of a cupshaped indicator 51. Cup shaped indicator 51 is normally disposed withinan axial bore 52 in end piece 11R and is urged outward therefrom to avisible position by a compression spring 54. Indicator wire 47 normallyholds indicator 51 in the inward position wherein spring 54 iscompressed between the bottom wall 50 of indicator 51 and the end plate23. Indicator 51 has a circumferential flange 56 adjacent its open end,and an eyelet 57 secured by suitable means such as staking to the endpiece 11R adjacent the margin dening axial bore 52 interferes withflange 56 and prevents indicator 51 from being removed from end piece11R.

Disposed within the interior of casing 10 and embedding the spider 26,the main ribbons'37, the auxiliary element 41 and the indicator wire 47is a body of granular inert or refractory material 55 of high dielectricstrength such as sand or nely divided quartz.

Under currents of short circuit magnitude, the main element 36 and theindicator wire 47 vaporize almost instantaneously, thereby permittingspring 54 to urge indicator 51 to a visible position as an indicationthat the 'fuse has operated. The disclosed fuse is particularly adaptedto clear relatively small magnitude currents of overload proportions,and FIGS. 4a-4f schematically show the sequence of operation under theseconditions and illustrate the main and auxiliary elements as a singleribbon 37 and as single wire 41 respectively to facilitate theunderstanding of the invention. Under relatively small but prolongedoverloads, the main ribbon 37 melts first at the low melting temperaturealloy bead 4t) near the center of the ribbon, and the main ribbon 37begins to burn back under the initial arc formed at this point as shownschematically in FIG. 4b. As discussed hereinbefore, even when the mainribbon 37 has perforations 39 forming portions of reduced crosssectional area, the arc gap first formed at the center of the ribbon isbeing progressively enlarged by the vaporization of the ribbon ratherthan forming arclets in series. The full line potential appears acrossthe glass-like fulgurite resulting from the fusion of the quartz filler55 immediately adjacent the initial arc, and the relatively large arcenergy, the excessive duration of arcing, and the 12R losses due to flowof follow current when clearing prolonged low magnitude current delaythe cooling of the central portion of the fulgurite so that it remainssemiconducting. Consequently, only the two end portions of the fulguritewhere the arc contacts the relatively cool quartz sand tend toextinguish the arc.

As. soon as the voltage rises sufiiciently across the arc formed at bead40 adjacent the center of the main element ribbon 37, the air gap 58between the clip electrode 43L shown at the left of the drawing and themain ribbon 37 sparks over, thereby causing current iiow through theindicator wire 47. Since the portion of indicator wire 47 within casing10 and embedded in the quartz sand liller 55 has better heat dissipationcharacteristics than the portion of indicator wire surrounded by airwithin axial bore 52 in end cap 11R, the latter portion surrounded byair will melt Ifirst under low fault conditions and assure movement ofindicator 51 to visible position.

Since the indicator wire 47 is of relatively high resistance, it willmelt almost immediately at a number of points and impose the arc voltageacross the air gaps 58 and 59 in series and flashover air gap 59 asshown in FIG. 4c and thus connect auxiliary element 41 in parallel withmain ribbon 37. The intense heat of the arcing vto the main element 37at the air gaps 58 and 59 quickly burns open sections of the main ribbon37 adjacent the clip electrodes 43L and 43R as shown in FIG. 4d.Auxiliary element 41 has been selected so that its minimum melt currentis approximately onefourth that of the main fusible element 37, andconsequently several half cycles will be required to vaporize theauxiliary element 41 on the relatively low fault current. During thetime required to melt the auxiliary element 41, arcing Will continue atthe air gaps 58 and 59 between the are gap electrodes 43 and the mainelement ribbons 37 and will destroy a small section of each main ribbon37 adjacent each of the air gaps. During the plurality of cycles theauxiliary element 41 is melting, the fulgurite at the center of the mainribbon 37 has had time to cool and lose its conductivity. Interruptionof the fault current by the auxiliary element 41 as schematicallyillustrated in FIG. 4f is particularly easy when the spider 24 .is ofgas evolving material which cools the inert granules 55 and helpsprevent reignition of the arc as disclosed in my aforementionedapplication S.N. 313,640. However, if insuficient length of the mainsilver ribbon 37 has been consumed, re-ignition of the main element 37may occur. As illustrated in FIG. 4f, the main ribbon 37 will now beburning back at the fulgurite at the center of the ribbon and also atthe fulgurites adjacent the air gaps 58 and 59. Since arcing isoccurring at three regions rather than at a single fulgurite, theburning back of the main ribbon 37 will occur at a much faster rate andthe arc energy is dissipated in three regions rather than at a singlefulgurite, thereby permitting the central portion of the fulgurites tocool and become a nonconductor. Further, the fuse will thus have the twoend portions with good arc extinguishing characteristics at each of thethree fulgurites, and consequently the disclosed lfuse rapidly clearssmall overload currents.

If the cross section of auxiliary element 41 is too large as compared tomain ribbon 37, the arc at the air gaps 58 and 59 may exist for asufficient length of time to damage that section of the fuse. If, on theother hand, the cross sectional area of auxiliary element 41 isinsufficient, the time of arcing at the air gaps may be too short tocompletely burn the main ribbon 37 open at this point and consequentlyonly one opening will be provided in the main ribbon if re-ignitionoccurs.

FIGS. 5 and 6 illust-rate embodiments wherein the main fusible elementis caused .to burn back at a still greater number of points whensubjected to small prolonged overload currents. The main and auxiliaryfusible elements of the embodiments of FIGS. 5 and 6 are preferablyWound helically on a support spider and embedded within a granular inertfille-r in the same manner as in the preferred embodiment, but in `orderto simplify the descri-ption and facilitate the understanding of theinvention, the main and auxiliary elements are shown in linear form inthe drawing and the arc gap electrodes are designated as arrowheads. Themain fusible element of the embodiment of FIG. 5 is a silver ribbon 37connected Ibetween the fuse end pieces (not shown) and having aplurality of spaced apart circular perforations 39 and a bead 40 of lowmelting temperature alloy adjacent its center. The serial arrangement ofthree silver wire auxiliary elements 61, 62, 63 is parallel to butelectrically isolated from the main ribbon element 37. The outerIauxiliary elements 61 and 63 may be of the same wire size and are ofsmaller wire size than the central auxiliary element 62. Electrodes 66and 67 at the outer ends of auxiliary elements 61 and 63 (which Imay besimilar to clip electrodes 43 of the preferred embodiment) form air gapswith the main electrode 37 which yare smaller than the air gaps formedbetween main ribbon 37 and electrodes 69 and 70 connected respectivelyat the junctions of auxiliary elements 61 and 62 and of auxiliaryelements 62 and 63. On small overload currents, the main ribbon 37 willfirst melt at the bead 40 at the center of the ribbon 37 and the ribbonwill burn lback from this point. After the arc voltage across thefulgurite at the center of ribbon 37 rises to a suiciently highmagnitude, the air gaps between ribbon 37 and electrodes 66 and 67flashover and continue to burn back the ribbon 37 at these -pointsduring the period required .to melt the auxiliary elements 61 and 63land thus clear the overload current. If the main ribbon 37 isre-ignited, the voltage across the 'ar-c lat the fulgurite adjacent thecenter of the main element 37 will rise sufficiently to flashover theair gaps between the main' lribbon 37 and the electrodes 69 and 70, andthe arcing at these air gaps during the period required to melt theauxiliary element 62 will destroy sections of the main ribbon 37adjacent the electrodes 69 and 70. If the main ribbon element 37 is nowre-ignited, live arcs are connected in series throughout the length ofmain element 37, and the main element will be burning back at tendifferent places, thereby providing ten fulgurite end portions with goodarc extinguishing characteristics which rapidly clear the overloadcurrent.

In the embodiment of FIG. 6, three auxiliary silver wire elements 71, 72and 73 are disposed parallel to main ribbon 37 Ibut isolated therefromby air gaps formed between main element 37 and electrodes 74 and 75connected to the ends of auxiliary element 71, electrodes 76 and 77connected to the ends of auxiliary element 72, and electrodes 78 and 79connected to the ends of auxiliary element 73. The center auxiliaryelement 72 is not joined to the outer auxiliary elements 71 and 73 as inthe embodiment of FIG. 5, and the center auxiliary element 72 crossesthel outer auxiliary elements 71 and 73 so that electrodes 76 and 77 arecloser to the ends of main ribbon 37 than electrodes 75 and 78. When themain element 37 melts adjacent bead 40 on a small fault current, the arcvoltage appears across only ele-ctrodes 76 and 77, thereby causing theair gaps between these electrodes `and the ribbon 37 to ashover. Thearcing at the air gaps causes the main ribbon .to melt adjacentelectrodes 76 and 77 during the interval required to melt the centerauxiliary element 72. If current flow should re-ignite the main element37 which has been burned open at three places, a portion of the larcvoltage will now appear across electrodes 74 and 75 and a portion of theare voltage will also appear across electrodes 78 and 79, therebycausing the air gaps between these electrodes and the main fusibleelement 37 to flashover. The subsequent arcing at these gaps causes themain ribbon 37 to melt adjacent arc gap electrodes 74, 75, 78 and 79during the interval required to melt the auxiliary elements 71 and 73.If current now re-ignites the main element 37, seven arcs will beconnected in series throughout the length of .main element 37, andfourteen fulgurite end portions having good arc extinguishingcharacteristics tend to interrupt the overload current.

While only Ia few embodiments of the invention have been illustrated anddescribed, many modifications and variations Ithereof will be readilyapparent to those skilled in the art, and consequently it is intended inthe appended claims to 4cover all such modifications and variationswhich fall within the true spirit and scope of the invention.

I claim:

1. In a high voltage fuse of the current limiting type, a tubularinsulating casing, metallic terminals on the ends of said casing, a mainfusible element within said casing interconnecting said metallicterminals, a body of low melting temperature alloy in intimate cont-actwith said main element, an auxiliary fusible element within said cahsrnghaving its ends -spaced slightly from portions of sald main element onopposite sides of said body of alloy and forming air gaps with said mainelement, the ratio of the 100 second melting current of said mainelement to the 100 second melting current of said auxiliary elementbeing in the range of from 2 to 6, and granular inert refractory arcquenching material within said casing ernbed-ding said main andauxiliary elements, whereby Hashover of said air -gaps connects saidauxiliary element in parallel with said main element and burns awayportions of said main element adjacent said air gaps while saidauxiliary element is melting and provides a plurality of arcing pointsin series along the length of said main element.

2. In an electric circuit interrupter capable of substantially limitingthe magnitude of fault current flowing therethrough including a fusiblemain element having a body of low melting temperature alloy in intimatecontact therewith, means for causing said main element .to burn back ina plurality of serially related points when interrupting Ilow magnitudefault current including an auxiliary fusible element spaced apart at itsends by Iair gaps from portions of said main element on opposite sidesof said body of alloy, the ratio of the 100 second melting current ofsaid main element to the 100 second melting current of said auxiliaryelement being in the range from 2 to 6, and granular inert arc quenchingmaterial embedding said main and auxiliary fusible elements.

3. In an electric circuit interrupter capable of substantially limitingthe magnitude of fault current flowing therethrough including a fusiblemain element having a 10W melting temperature `alloy element in intimatecon- -tact therewith, means for causing said main element to burn backin a plurality of serially related points when interrupting lowmagnitude fault current including an auxiliary fusible element spacedapart at its ends by air gaps from portions of said main element onopposite sides of said alloy element, means disposed adjacent each saidair gap and responsive to arcing in said gap for burning completelythrough a portion of said main element adjacent said gap, and granulararc quenching material ernbedding said main and auxiliary fusibleelements and said last-named means.

4. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casing, a main fusibleelement within said casing interconnecting said terminals and having amaximum long time melting temperature of approximately 600 F. whensubjected to prolonged fault currents of relatively small magnitude, anauxiliary fusible element within said casing having its ends spacedslightly by air gaps from longitudinally spaced apart portions of saidmain element, the ratio of the second melting current of said mainelement to the 100 second melting current of said auxiliary elementbeing in the range from 2 to 6, and granular inert refractory arcquenching material within said casing embedding said main and auxiliaryelements, whereby ashover of said air gaps connects said auxiliaryelement in parallel with said main element and burns away portions ofsaid main element adjacent said air gaps, while said auxiliary elementis melting, and provides a plurality of arcing points in series alongthe length of said main element.

5. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casing, a main fusibleelement within said casing interconnecting said terminals, a body of lowmelting temperature alloy in intimate contact with said main element, anauxiliary fusible element within said casing having its ends spacedslightly from portions of said main element on opposite sides of saidbody of alloy forming air gaps with said main element, the ratio of the100 second melting current of said'main element to the 100 secondmelting current of said auxiliary element being in the range from 2 to6, insulating support means within said casing for said main and fusibleelements and being in contact with said fusible elements at only spacedapart points along the length of said main and auxiliary elements andbeing adapted to evolve gas in the presence of an arc, and granularinert refractory arc quenching material within said casing embeddingsaid support means and said main and auxiliary elements.

6. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casings, a spiderextending parallel to the axis of said casing and having peripherallyspaced apart, radially protruding fins generally longitudinal thereof, amain fusible element within said casing interconnecting said terminalsand being wound helically on said spider so that it touches only saidns, a body of low melting temperature alloy in intimate contact withsaid main element, an auxiliary fusible element within said casing woundhelically on said spider so that it touches only said tins and havingits ends spaced slightly from portions of said main element on oppositesides of said body of alloy and forming air gaps with said main element,the ratio of the 100 second melting current of said main element to the100 second melting current of said auxiliary element being in the rangefrom 2 to 6, the portions of said tins in contact with said main andauxiliary elements being of an insulating material adapted to evolve gasin the presence of an arc, and granular inert refractory arc quenchingmaterial within said casing embedding said spider and said main andauxiliary elements.

7. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends 0f said Casing, a spider ofstar shaped cross section having radially protruding fins extendinglongitudinally thereof disposed within said casing parallel to the axisthereof, a main fusible element within said casing interconnecting saidterminals and being helically wound on said spider and touching onlyspaced apart portions of the periphery of said spider along said fins, abody of low melting temperature alloy in intimate contact with said mainelement, an auxiliary fusible element within said casing wound helicallyon said spider and touching only spaced apart portions of the peripheryof said spider along said fins and having its ends spaced slightly fromportions of said main element on opposite sides of said body of alloyand forming air gaps with said main element, the ratio of the 100 secondmelting current of said main element to the 100 second melting currentof said auxiliary element being in the range of two to six, granularinert refractory material of high dielectric strength within said casingembedding said spider and said main and auxiliary elements, said spiderbeing of a molded, thermosetting, electrical insulating composition,including a water insoluble binder and an anti-tracking material adaptedto evolve gas when heated by an arc selected from a class consisting ofthe hydrates and oxides of magnesium and aluminum.

8. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casing, a spider ofheat resistant insulating material extending parallel to the axis ofsaid casing having peripherally spaced apart, radially protruding finsgenerally longitudinal thereof, said fins having depressions spacedapart longitudinally thereof forming raised portions between saiddepressions and the depressions of peripherally successive iinsadvancing longitudinally of said spider and defining a generally helicalpath and said raised portions of peripherally successive fins forminggenerally helical support means radially outward from said helical path,a main fusible element within said casing interconnecting said terminalsand being wound on said helical support means defined by said raisedportions, a bead of low melting temperature alloy in intimate contactwith said main element, anauxiliary fusible element within saidcasingdisposed in the helical path defined by said depressions andhaving its ends spaced slightly from portions of said main element onopposite sides of said bead forming air gaps with said main element, theratio of 100 second melting current of said main element to the 100second melting current of said auxiliary element being in the range fromtwo to six, and granular inert refractory arc quenching material withinsaid casing embedding said spider and said main and auxiliary elements.

9. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casing, a spider ofheat resistant insulating material adapted to evolve gas in the presenceof an arc extending parallel to the axis of said casi-ng and havingiirst and second concentric spaced apart, generally helical paths in theexterior periphery thereof, a main fusible element within said casingdisposed in said first helical path and interconnecting said terminals,a body of low melting temperature alloy in intimate contact with saidmain fusible element, an auxiliary fusible ele-ment within said casingdisposed in said second helical path and having its ends spaced slightlyfrom portions of said main element on opposite sides of said alloy bodyand forming air gaps 'with said main element, the ratio of the 100second melting current of said main element to the 100 second meltingcurrent yof said auxiliary element being in the range of from two tosix, said spider co-ntacting only portions of said main and auxiliaryelements spaced apart longitudinally of said elements, and granularinert refractory arc quenching within said casing embedding said spiderand said main and auxiliary elements.

x10. In a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals onthe ends of said casing, a spider of heatresistant insulating material adapted to evolve gas in the presence ofan arc extending parallel to the axis of said casing having peripherallyspaced apart, radially protruding -ins generally longitudinal thereof,said tins having depression spaced apart longitudinally thereof formingraised portions be- CII tween said depressions and the depressions ofperipherally successive fins advancing longitudinally of said spider anddefining a genrally helical path and said raised portions ofperipherally successive .ns forming generally helical support meansradially outward from said helical path, a main fusible element withinsaid casing interconnecting said terminals and being wound on saidhelical support means defined by said raised portions, a bead of lowmelting temperature alloy in intimate contact with said main element,arc gap electrodes atlixed to individual raised portions of said spidero-n opposite sides of said bead and being spaced slightly inwardradially from said main elements and defining air gaps therewith, anauxiliary fusible element within said casing disposed in the helicalpath defined lby said depressions and bein-g affixed at its ends to saidarc gap electrodes, the ratio of second melting current of said mainelement to the 100 second melting current of said auxiliary elementbeing in the range from two to six, and granular inert refractory arcquenching material within said casing embedding said spider and saidmain and .auxiliary elements and said arc gap electrodes.

11. In an electric circuit having current limiting fuse means forsubstantially limiting the magnitude of fault current flowing throughsaid circuit including a fusible main element having a body of lo'wmelting temperature alloy in intimate contact therewith and granularinert arc quenching material embedding said main element, means forcausing said main element to burn back in a plurality of seriallyrelated points Iwhen interrupting low magnitude fault current includingan auxiliary element embedded in said arc quenching material andextending parallel to said main element and spaced apart at its ends by`first and second air gaps `from portions of said main element onopposite sides of said body of alloy, and a second auxiliary elementembedded in said arc quenching material and extending parallel to saidmain element and having its ends spaced slightly from portions of saidmain element on opposite sides of said iirst air gap and forming thirdand fourth air gaps with said main element, whereby the arc voltagedeveloped across the portion of said main element burned away by arcingat said first air gap will ilashover said third and fourth air gaps.

12. In combination with an electric circuit having current limiting fusemeans for substantially limiting the magnitude of fault current flowingthrough said circuit including a fusible main element having a lowmelting temperature alloy element in intimate contact therewith, meansfor causing said main element to` burn Iback in a plurality of seriallyrelated points when interrupting low magnitude fault current includingthree serially connected auxiliary fusible elements extending parallelto said main element, the middle auxiliary element having a higherminimum melting current than the outer auxiliary elements, the junctionsof said three auxiliary elements being spaced apart by first and secondair gaps from portions of said main element on opposite sides of saidalloy element and the outer ends of said outer auxiliary elements beingspaced rfrom said main element by third and fourth airgaps spacedfurther from said alloy element than said first and said second air gapsand having lower flashover voltages than said iirst and second air gaps,and granular inert arc quenching material ernbedding said main and saidauxiliary fusible elements.

13. lIn a high voltage fuse of the current limiting type, a tubularinsulating casing, terminals on the ends of said casing, a main fusibleelement within said casing interconnecting said terminals, a body of lowmelting temperature alloy in intimate contact with said main element, anauxiliary fusible element within said casing having its ends spacedslightly from portions of said main element on opposite sides of saidbody 0f alloy and forming air gaps with said main element, meansdisposed adjacent each said air gap and responsive to arcing in `saidair gap -for burning through a portion of said main element adjacentsaid air gap, land inert granular refractory arc quenching materialwithin said casing embedding said main and auxiliary elements and saidlast-named means, whereby the are voltage across the portion of saidmain element at said body of alloy 12 when interrupting 10W magnitudefault current will flashover said air gaps `and burn iback said mainelement adjacent each said air gap.

No references cited.

B, A. GILHEANY, Primary Examiner.

H. B. GILSON, Examiner.

2. IN AN ELECTRIC CIRCUIT INTERRUPTER CAPABLE OF SUBSTANTIALLY LIMITINGTHE MAGNITUDE OF FAULT CURRENT FLOWING THERETHROUGH INCLUDING A FUSIBLEMAIN ELEMENT HAVING A BODY OF LOW MELTING TEMPERATURE ALLOY IN INTIMATECONTACT THEREWITH, MEANS FOR CAUSING SAID MAIN ELEMENT TO BURN BACK IN APLURALITY OF SERIALLY RELATED POINTS WHEN INTERRUPTING LOW MAGNITUDEFAULT CURRENT INCLUDING AN AUXILIARY FUSIBLE ELEMENT SPACED APART AT ITSENDS BY AIR GAPS